Phased array scanning antenna

ABSTRACT

A landing system antenna in which a plurality of antenna elements arranged in a circular pattern are sequentially energized to project a scanning planar beam with fine scanning control maintained by controlling the phase in a plurality of small steps.

United States Patent Schwartz et al. June 3, 1975 [54] PHASE!) ARRAY SCANNING ANTENNA 3.056.961 ll/l962 Mitchell 343/854 3,568,207 3 I971 B t l [751 'Vemors: schwam; Scarsdalfi? 3,653,057 7/1972 c3131; 343/854 Edward Rego Park 3.775.769 11/1973 Heeren et a1. 343/1154 both of NY [73} Assignee: The Singer Company, Little Falls, Primary Examiner Eli Lieberman Attorney, Agent, or FirmThomas W. Kennedy [22] Filed: Nov. 14, 1973 [21] Appl. No.: 415,523 [57] ABSTRACT [52] US. Cl. 343/816; 343/837; 343/854 A landing system antenna in which a plurality of an- {51} Int. Cl. HOlq 3/26 tenna elements arranged in a Circular P are 581 Field of Search 343/840, 854, 816, 837 quemially energized to P j a scanning Planar beam with fine scanning control maintained by controlling 5 References Cited the phase in a plurality of small steps.

UNITED STATES PATENTS 7 Claims, 12 Drawing Figures 2,523,455 9/!950 Stewart 343/7925 REFLECTOR DIPOLES COAX FEED J LINE DE FOCUSED l PARABOLIC ARC HEET

PATENTED 3 OmEmmQ PATENTED 3 SHEET OPTIMUM BiNOMlAL UNIFORM PATENTEDJm I975 3.887.926 sum 3 UNIFORM PHASE FRONT DIRECTION OF BEAM PEAK PATENTEUJUM3 m5 3, 887'. 926 SHEET 4 3 dB CONTOUR 52 REFLECTOR DIPOLES COAX FEED LINE DE FOCUSED PARABOLIC ARC PATENTEDJUNIs 1915 3,887,926

PHASED ARRAY SCANNING ANTENNA BACKGROUND OF THE INVENTION This invention relates to antennas in general and more particularly to an antenna useful in aircraft landing systems.

Landing systems presently in use generally transmit scanning beams to provide the pilot in the aircraft with glide slope and localizer information. The nature of these systems is such that the information available to the pilot is for the most part only whether or not he is on the proper heading and glide slope or the direction in which he is off that path. The advantages ofa landing system which would provide more information to the pilot and would also aid in automated landings has been recognized. A system under consideration for future use would provide a localizer beam and a glide slope beam which would be scanned over a relatively large area. While scanning, the beams would be modulated to provide information regarding the instantaneous direction in which the beam was pointing. In this manner, a pilot anywhere within a large volume defined by the glide slope and localizer beams would be able to determine his position exactly with respect to the touchdown point. This would permit better correction in preparation for landing and would also make available the possibility of landing on a line other than the extension of the runway center line. Such a capability may be particularly useful in certain crosswind situations and may be required to reroute aircrafts in order to abate noise pollution. Such a system requires the antennas capable of scanning the required volume. Typically, the azimuth antenna would be required to scan over plus and minus 60 and the elevation or glide slope antenna from -l to 20. To provide the type of accurate information required, each antenna would be required to provide a planar beam with a very narrow beam width for example, approximately 1. It is well known in the antenna art that the narrower the beam width, the large the antenna required. The most obvious solution to providing a scanning planar beam is to mechanically scan the antenna. However, where a large size antenna is required as in the present case, and a relatively high scanning rate must be provided, the associated mechanical problems become extremely difficult. This is particularly true in this case since the antenna must be scanned at a rate of H2. To accurately move a large antenna at this scan rate becomes a serious problem and may not be a practical possibility. It is possible to scan a beam using a planar antenna array by controlling the phase in the various elements of the array, however, such a scan produces a pattern which is not planar over its complete scan volume and thus does not meet the requirements of the system. Thus, there is a need for an electronically scanned antenna which has a beam which is planar over the total scan volume and which is accurately positioned at all times.

SUMMARY OF THE INVENTION The antenna of the present invention meets the requirements of the landing system by providing a plurality of antenna elements arranged in a circular pattern. A plurality of the elements are energized to project a planar beam in the desired direction, with the phase of each element being controlled to present a uniform phase front perpendicular to the scan direction. Coarse scanning is provided by switching in and out the elements at each end of the radiating group. Fine scanning is provided by controlling the phase in a plurality of small steps when in between two elements of the array. Optimum performance is obtained by focusing the beam out of the plane of the radiating elements, thereby minimizing the inherent beam width broadening outside of the principal plane of focus. Amplitude of the various elements is also controlled to further control beam width and reduce side lobes. Both ampli' tude and phase shift are controlled digitally by a preprogrammed digital computer controlling PIN diode devices. Two arrangements are shown, one using a plurality of dipoles and focusing devices, and another comprising a plurality of linear arrays.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view illustrating the type of beams required in a micro-wave landing system such as that disclosed herein.

FIGS. 2A and 2B illustrate the difference between planar beam scanning and conical beam scanning.

FIG. 3 illustrates the manner in which beam width and side lobes may be controlled using a plurality of radiating elements.

FIG. 4 illustrates a circular array according to the present invention showing the manner in which switching between elements takes place.

FIG. 5 illustrates the fine scanning of the present invention.

FIG. 6 illustrates a beam pattern for a beam having its phase focused in the plane of the antenna.

FIG. 7 illustrates a similar view of a beam which is focused out of the plane of the antenna to maintain more constant beam width.

FIG. 8 is a block diagram of the control arrangement of the present invention.

FIG. 9 is a perspective view of an hourglass antenna useful in the present invention.

FIG. 10 is a perspective view ofa dipole antenna and reflector according to the present invention.

FIG. 11 is a perspective view of a wave guide array antenna according to the present invention. See end of the figure description.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 which illustrates the types of beams which must be provided by the antenna of the present invention is helpful in understanding the requirements placed on the antenna. The azimuth antenna will be located at a point 11 along the runway center line I3 and must provide a beam 15 which has a narrow beam width in the horizontal direction and scans horizontally within the required limits. The glide slope antenna will be located at a point 17 to provide a beam 19 which is narrow in the vertical plane and must scan vertically between limits as shown. The beam 15 will be modulated to provide coded information as to its angle from the center line 13 of the runway and beam 19 coded to provide its deviation from a desired glide slope 21. Thus an aircraft, for example, at a point 23 may receive these beams and therefore determine the azimuth steering error 25 and an elevation steering error 27. These may then be used to make necessary corrections in the control of the aircraft to follow the proper glide path. Also shown is a conically scanned flare antenna pattern 29 and a departure guidance scan 31, which are part of the total proposed system, but will not be discussed herein.

The need for the beams and 19 to be planar beams is best illustrated by FIG. 2. FIG. 2A illustrates the nature of the more standard conical beam scanning and FIG. 28 illustrates planar beam scanning. In conical beam scanning the beam falls on the equator 31 when in the plane of the antenna, but as it is scanned upward, it will fall on lines of latitude as shown by beam 19a. In planar beam scanning, however, the beam 19 will always lie along a great circle passing through the sphere as shown by beam 19b. The information which is desired to be presented to the aircraft is the glide slope angle which will be the angle of the plane formed by the great circle 33. It will be seen that with conical beam scanning this information will be provided erroneously at any point off the center line 35. Thus, to provide this information accurately a planar beam scan is required. What is shown here for the glide slope scan is equally true in the azimuth or localizer scan. With other than a planar beam, the information at the ends of the beam will be in error.

Another requirement of the beams provided in such a system is that the beam does not have high side lobe levels. This is necessary particularly in elevation to prevent ground reflections which could provide erroneous information to the aircraft. As mentioned previously, it is well known that to provide a narrow beam a large antenna is required. FIG. 3 illustrates the manner in which side lobes ofa beam can be controlled. As an ex ample, S isotropic point sources 41 are shown. With the amplitude at each of the sources equal, as indicated by the ones below the sources, a beam which is relatively narrow, but which has significant side lobes results. As shown in the second example of FIG. 3, a binominal distribution will provide a beam which has almost no side lobes but which is wider. The third example shows what is considered to be an optimum beam in which the narrowness is improved and the side lobes decreased, i.e. it is a compromise of the first two examples. As will be seen below in the discussion of the preferred embodiment, the amplitude is controlled to provide mini mum side lobes for the desired beam width.

Note that two separate cylindrical arrays will be required, one for the azimuth scanning antenna and one for the elevation scanning antenna, the glide slope antenna. In theory, the two antennas are identical. All that is required is to rotate the elevation antenna 90 relative to the azimuth antenna. Thus, the two azimuth antennas shown in FIG. 9 and in FIG. 11 rotated 90 will represent the preferred embodiments for the elevation antennas. Because of this duality, only a discussion of the azimuth version of the cylindrical array will be given in detail. Although identical antennas may be used for azimuth and elevation, in practice, the design of the elevation antenna will be slightly different. For example, as illustrated in FIG. 1, the area scanned in azimuth is considerably larger than the area scanned in elevation and, consequently, the arc sector for the azimuth antenna will naturally be larger than that for the elevation antenna. A cross-section of a cylindrical array arrangement is illustrated on FIG. 4. A plurality of elements 43 are arranged in circular fashion. To obtain the narrow beam required, a plurality of these elements 43 will be required to radiate at any given time. In addition, to provide the proper beam shaping, they where: 5(11) the phase correction required for the array at an angle 1/ on the circle, A the operating wave length R the radius of curvature of the circle and d1 the angle in the plane of the circle where the in-phase condition is desired.

Outside the plane of the circle in FIG. 4, the phase correction required is given by:

where 6 the angle measured from the plane of the circle. It is obvious that equation (2) reduces to equation (1) in the plane of the circle where 6 equals zero degrees. Thus, by controlling the phase of the various elements a beam in a given direction may be provided by the circular array. Coarse scanning may then be provided by sequentially switching between the elements of the circular pattern. That is, for example, when going from the pattern indicated collectively as 51 to that indicated as 53, the last element 43a would be switched out and the next element on the other end, 43b switched in, at the same time adjusting the phase of the various elements. As noted above, to provide the required accuracy, beam steps must be very small. It is not practical to provide a number of elements equal to the number of steps. The system of the present in vention overcomes this problem in a unique way. As shown on FIG. 5, assume that 5 elements are radiating in a direction from the center 57 of the antenna passing through an element 430 to a uniform phase front 450. The next major step would be a wave front in the direction passing through element 43d to a phase front 45b. If the antenna is to move in small steps, phase fronts such as 450 will have to be provided between 45a and b. This may be done by assuming an imaginary radiator 59 between 2 of the radiators 43 and then computing the phases with respect thereto for the radiators 43. In this manner a series of small steps, each represented by an imaginary radiator, through which a line perpendicular to a uniform phase from is drawn. These imaginary radiators at the required spacing may be used to compute the phase for each of the elements and thus provide a series of small discreet scanning steps. In practice, the required phases to scan these small discrete steps can be generated by a digital computer using equation l) for each angle 42 Upon generation of the required phases, the appropriate phase control signals 77, shown in FIG. 8, will set the digital phase shifters 67 to provide the beam shift 450 or 45b. After a predetermined number of fine step scans, the last element 43a would be switched out and the next element on the other end, 43b switched in. The process for the fine step scans will then be repeated.

If the size of the step is close to the noise level of the receiving system, the fine step scanning antenna cannot be differentiated from a true continuously scanning antenna. Obviously, high resolution phase shifters would be required. It can be shown, however, that steps if the order of 0.0l50 can be generated using 5. bit digital phase shifters. A n-bit phase shifter is a phase shifter capable of generating 2 n discrete phase intervals per 360 phase cycle. The greater the number of hits the more accurate will be the scanning process. For example, it can be shown that for a 0.0l5 fme step scan, the average beam pointing error is zero in the plane of the circle.

What has been shown on FIGS. 4 and 5 is a plan view of a beam which is pointing in the direction 61 and in that direction forms a narrow beam such as shown on FIG. 1. What must be realized is that in the other dimension this beam is of a relatively large size. For example, refer to the elevation beam 19 of FIG. 1 which is quite narrow vertically but which has a large horizontal width. The beam formed by the antenna in the direction 61 must be focused at some point. The most natural place to focus this beam would be in the plane of the antenna. The result of such focusing is shown by FIG. 6 where 6 is deviation from the antenna plane and lines 62 represent beam width. In the plane of the antenna, the beam will be very narrow, but as the edges are approached the beam widens. The reason for this is that in the plane of the circle, equation l can be im plemented exactly by the phase shifters 67. However, the correction outside of the plane of the circle requires a cos 6 factor as shown in equation (2). Once the phase shifters are set, it is obvious that the phase correction out-of-phase will be in error and this error will continue to increase as 9 increases. Thus, the beam width 62 of the antenna will broaden out of plane as shown in FIG. 6. Obviously, it is desirable to keep the beam width somewhat uniform, at least over the range at which it is expected to be intercepted by the aircraft. It has been found that this may be accomplished by focusing the beam out of the plane of the antenna. That is to say, when determining the direction of the uniform phase front as shown on FIGS. 4 and 5 a uniform phase front which is at some angle displaced from the plane of the beam will be computed. The pattern for any known antenna can be computed using well-known computer programs. Thus it is possible to progress through a series of possible out of plane angles to determine which of these will give the most uniform beam width within the desired range. As a practical matter, this means that instead of solving equation (l) to set the phase shifters, equation (2) instead is used for some fixed angle 6 which has been selected to minimize the error over the range of 9 angles desired.

By minimizing the phase errors out of plane, we can minimize the change in beam width out of plane. As an example, it can be shown that for a 128 element antenna curved into a arc with a radius of curvature of 425 inches, the beam width broadening over :40 out ofplane is almost 33 percent. This intolerable beam width broadening is associated with all cylindrical arrays. In US. Pat. No. 3,653,057 granted to G. G. Charlton on Mar. 28, 1972, this broadening was minimized by using a multi-layer feed matrix. In comparison, we have shown that the beam width broadening over :40" can be reduced to about 6 percent by correcting for the phase error at about out-of-plane. This is illustrated in FIG. 7 where we have attempted to show that the 3-DB contours, over a relatively large angular range out-of-plane, remained relatively constant as opposed to FIG. 6 where the beamwidth broadens very rapidly out-of-plane over a much smaller angular region. As shown by equation (2), the phase correction function varies as the cosine of the angle out-of-plane. Since the cosine function is an even function, the phase errors, and thus the resultant beam width, will be symmetrical about the plane of the circle as shown in FIG. 7.

It will be recognized, that for any beam direction the required phase at each of the elements may be precomputed using well-known antenna equations to determine the phase needed to present a uniform phase front at a predetermined line. This makes it possible to precompute for each scanning step the phase of each of the elements and at what time which of the elements must be switched on. Precomputing the phase values would offer an economical alternative to computing the phases required in real time. In conjunction with this, it is also possible to precompute the necessary amplitude at each of the elements to minimize side lobes. This data may then be precomputed and formed into a table which may be stored in a computer. This table would then list for each incremental step through the scanning range of the antenna which elements must be turned on, the phase required for each of the elements, and the corresponding amplitude required. FIG. 8 illustrates a preferred embodiment of the control elements to control the amplitude and phase of each of the elements of the antenna. A beam controller 65 which would comprise a digital computer wherein are stored the precomputed tables along with clock means to increment the computer through the scanning steps is shown. Four of the radiating elements 43 are shown as an example. Associated with each of the elements 43 is a digital phase shifter 67 and an amplitude controller 69. Power is provided from a transmitter 71 to a divider power distribution block 73 then through an amplitude controller, a digital phase shifter, and then through a switching matrix to each of the elements 43. The digital phase shifter 67 and the amplitude controllers 69 will both be PIN diode devices which are digitally controlled. That is, they will respond to control the amplitude or shift the phase in response to an input digital word from the beam controller 65. The beam controller 65 will output, on a plurality of lines shown collectively as line 75, digital amplitude control signals to the aplitude controller 69 and similarly, on a plurality of lines indicated collectively as line 77, phase control signals to the digital phase shifters. They will also provide on lines 79 switching signals to switch between the elements 43 when passing through the large steps of scanning as described above. The digital information provided by beam controller 65 will be obtained from the aforementioned tables and will change for each step of the scan.

At this point it should be noted that although the localizer beam 15 shown on FIG. I is illustrated as having a uniform width across its full vertical direction, it is in practice desirable to have the beam widen at the top to aid in intersecting the beam when an aircraft is at at large distance from the runway. One manner of obtaining this beam shaping is illustrated by the embodiment of FIG. 9. In this embodiment, the antenna elements comprise dipoles 8! arranged in circular fashion as described above and fed by wave guides 83 from within a parabolic, or hourglass reflector 85. A reflecting ring 87 is placed around the dipoles to cause the radiated energy to reflect therefrom to the reflector 85 and thence be radiated in a focused and shaped manner as required. The shape of the hour glass reflector is generated by rotating a shaped reflector about the vertical axis. The precise shape of the reflector elevation crosssection is dependent on the shape of the radiation pattern desired in the elevation plane. The design and analysis is well known and a discussion can be found in Microwave Antennas Theory and Design" by Samuel Silver. Such shaping is not required of the beam pro jected by the glide slope or elevation antenna. Thus, as shown in FIG. 10, the elevation antenna could comprise a plurality of dipoles 89 backed by a reflector 91. In FIG. 10, each of the dipoles 89 arranged along a circular arc is connected to the element switching matrix shown in FIG. 8. The excitation of the elements 89 will form a narrow beam in the vertical plane. The reflector 91 behind the dipole elements focuses the beam pattern in the azimuth plane. As shown, because of the relatively small aperature for the reflector, the azimuth beam width will be fairly broad. In practice, the ideal azimuth beam width of the elevation antenna will be in the order of 120.

A second embodiment is shown in FIG. 11. In this embodiment, a plurality of linear wave guides 93 are formed into a circular array. Such wave guides, which are wellknown in the art and described in various patents and textbooks may have slits cut therein to form the wave to the desired shape. For example, see US Pat. No. 3,604,010 granted L. Schwartz on Sept. 7, l97l and assigned to the same assignee as the present invention for a discussion of the shaping of waves through the use of such linear wave guides. In FIG. 11, each of the waveguide arrays 93 are connected to the beam switching matrix of FIG. 8. Azimuth scanning is accomplished as explained in connection with FIG. 4. As explained in the above referenced patent, the arrangement of the slits in the waveguide in the vertical direction can be used to determine the radiation beam shape in the vertical plane. Thus, there is no need for a reflector as in FIG. 9.

This second embodiment, using waveguide arrays, can also be used as the elevation scanning antenna by rotating the antenna shown in FIG. 11 by 90. As explained in Silver and other texts on antennas, slits other than edge-cut shunt slots may be used. Thus, by appropriately changing the type of slots shown in FIG. 11, it is possible to obtain an elevation scanning antenna with either vertical or horizontal electric field polarization. The choice of the polarization desired will in turn depend on the system performance requirements.

Thus, a method and apparatus of providing a planar antenna beam which is scanned within predetermined limits has been shown. Although specific embodiments have been illustrated and described, it will be obvious to those skilled in the art that various modifications may be made without departing from the spirit of the invention which is intended to be limited solely by the appended claims.

What is claimed is:

l. A microwave landing system scanning beam antenna which provides a planar fan beam comprising:

a. a set of equally spaced radiating elements arranged to form one of a portion of a circle and a portion of a cylinder;

b. a source of microwave energy;

c. a plurality of phase shifters each coupling said source to one of said plurality of radiating elements;

d. a plurality of digital amplitude controllers one being interposed between the source and each phase shifter;

e. means to store digital data comprising:

1. a plurality of representations of radial lines equally spaced each representing an angular deviation from radial line through the center of an arc formed by said set of radiating elements, said plurality of representations dividing the angle equivalent to the spacing between two of said radiating elements into a plurality of small incre' ments;

2. a digital representation of the phase shift required for each of said phase shifters to present a uniform phase front perpendicular to each of said radial lines; and

3. a digital representation of the amplitude of each of said radial lines, and

f. means to sequentially provide as outputs to said phase shifters digital representations of phase shift associated with each of said radial lines, and to sequentially provide as outputs to said digital amplitude controllers and digital representation of amplitude associated with said radial lines, whereby the antenna beam will be scanned in small increments through an angle between two elements with minimum side lobes in the radiated beam.

2. The invention according to claim 1 and further including:

a. additional radiating elements arranged adjacent said set in similar fashion forming a plurality of elements;

b. an element switching matrix having the outputs of said plurality of phase shifters as inputs and having outputs to each element in said plurality of elements and responsive to a digital switching input to provide the inputs from said phase shifters to any contiguous set of elements equal in number to said set of phase shifters; and

c. said means to sequentially provide outputs further includes means to provide digital switching outputs to cause, after each stepping through increments equal to one element spacing, at new set of elements to be coupled to said phase shifters, with an element at one end of the former set being disconnected and a new element being connected at the other end whereby the antenna may scan a large volume in small increments with the coarse scanning provided by element switching.

3. The invention according to claim 2 wherein said phase shifts are selected to present a uniform phase front in a plane other than the plane of the arc in which said elements are contained, said other plane being a plane which optimizes the inherent deterioration of the antenna pattern.

4. The invention according to claim 2 wherein said elements comprise a plurality of dipoles arranged about a reflector.

5. The invention according to claim 2 wherein said elements comprise a plurality of dipoles supported by and fed from outside an hourglass reflector and further including a narrow cylindrical reflector surrounding said dipoles whereby said cylindrical reflector will re- 9 10 fleet energy to said hourglass reflector from which said shell. energy will be reflected outwardly with the phase re- 7. The invention of claim 2 wherein the antenna quired to present a constant phase front. wherein the beam is rotated orthogonal to the principal 6. The invention according to claim 2 wherein said axis of said array. elements are linear arrays arranged about a cylindrical 5 

1. A microwave landing system scanning beam antenna which provides a planar fan beam comprising: a. a set of equally spaced radiating elements arranged to form one of a portion of a circle and a portion of a cylinder; b. a source of microwave energy; c. a plurality of phase shifters each coupling said source to one of said plurality of radiating elements; d. a plurality of digital amplitude controllers one being interposed between the source and each phase shifter; e. means to store digital data comprising:
 1. a plurality of representations of radial lines equally spaced each representing an angular deviation from radial line through the center of an arc formed by said set of radiating elements, said plurality of representations dividing the angle equivalent to the spacing between two of said radiating elements into a plurality of small increments;
 2. a digital representation of the phase shift required for each of said phase shifters to present a uniform phase front perpendicular to each of said radial lines; and
 3. a digital representation of the amplitude of each of said radial lines; and f. means to sequentially provide as outputs to said phase shifters digital representations of phase shift associated with each of said radial lines, and to sequentially provide as outputs to said digital amplitude controllers and digital representation of amplitude associated with said radial lines, whereby the antenna beam will be scanned in small increments through an angle between two elements with minimum side lobes in the radiated beam.
 1. A microwave landing system scanning beam antenna which provides a planar fan beam comprising: a. a set of equally spaced radiating elements arranged to form one of a portion of a circle and a portion of a cylinder; b. a source of microwave energy; c. a plurality of phase shifters each coupling said source to one of said plurality of radiating elements; d. a plurality of digital amplitude controllers one being interposed between the source and each phase shifter; e. means to store digital data comprising:
 1. a plurality of representations of radial lines equally spaced each representing an angular deviation from radial line through the center of an arc formed by said set of radiating elements, said plurality of representations dividing the angle equivalent to the spacing between two of said radiating elements into a plurality of small increments;
 2. a digital representation of the phase shift required for each of said phase shifters to present a uniform phase front perpendicular to each of said radial lines; and
 2. The invention according to claim 1 and further including: a. additional radiating elements arranged adjacent said set in similar fashion forming a plurality of elements; b. an element switching matrix having the outputs of said plurality of phase shifters as inputs and having outputs to each element in said plurality of elements and responsive to a digital switching input to provide the inputs from said phase shifters to any contiguous set of elements equal in number to said set of phase shifters; and c. said means to sequentially provide outputs further includes means to provide digital switching outputs to cause, after each stepping through increments equal to one element spacing, a new set of elements to be coupled to said phase shifters, with an element at one end of the former set being disconnected and a new element being connected at the other end whereby the antenna may scan a large volume in small increments with the coarse scanning provided by element switching.
 3. a digital representation of the amplitude of each of said radial lines; and f. means to sequentially provide as outputs to said phase shifters digital representations of phase shift associated with each of said radial lines, and to sequentially provide as outputs to said digital amplitude controllers and digital representation of amplitude associated with said radial lines, whereby the antenna beam will be scanned in small increments through an angle between two elements with minimum side lobes in the radiated beam.
 3. The invention according to claim 2 wherein said phase shifts are selected to present a uniform phase front in a plane other than the plane of the arc in which said elements are contained, said other plane being a plane which optimizes the inherent deterioration of the antenna pattern.
 4. The invention according to claim 2 wherein said elements comprise a plurality of dipoles arranged about a reflector.
 5. The invention according to claim 2 wherein said elements comprisE a plurality of dipoles supported by and fed from outside an hourglass reflector and further including a narrow cylindrical reflector surrounding said dipoles whereby said cylindrical reflector will reflect energy to said hourglass reflector from which said energy will be reflected outwardly with the phase required to present a constant phase front.
 6. The invention according to claim 2 wherein said elements are linear arrays arranged about a cylindrical shell. 